1. Field of the Invention
The present invention relates to a process for the production of monoblocked diisocyanates and the products thereof.
2. Discussion of the Background
Complete blocking of isocyanates for temporary protection of their NCO groups has been known for a long time and is carried out on a technical scale, e.g., in the production of 1-K-PUR powder or 1-K fired enamels which contain solvents. A 1-K-PUR powder is a powder containing a blocked isocyanate and a hydroxyl group containing compound. 1-K fired enamels additionally contain a solvent. These mixtures are stable at room temperature. However, when heated the blocking group splits off and a reaction between the isocyanate and the hydroxyl groups occurs.
Monoblocking of diisocyanates, i.e., using one mole of monofunctional blocking agent per mole of diisocyanate, is also known to the art. Monoblocked diisocyanates are useful in the manufacture of polyisocyanate-urea-adducts as disclosed in copending U.S. Pat. application Ser. No. 07,245,217, filed Sep. 16, 1988, which is incorporated herein by reference.
The process for the production of these monoblocked diisocyanates involves the following steps: (i) adding the blocking agent to the diisocyanate, which may be dissolved in an inert solvent, at room temperature up to 120.degree. C., in increments, and (ii) heating this mixture until the calculated NCO content is reached. For example, according to DE-OS 31 43 060, an isophorone diisocyanate monoblocked with epsilon caprolactam is produced by the reaction of 1 mole of isophorone diisocyanate (IPDI, also called 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate) and 1 mole of epsilon caprolactam at 120.degree. C.
An IPDI monoblocked with epsilon caprolactam produced in this way still contains approximately 16% non-converted IPDI, however This is easily understandable, since with an equimolar mixture of IPDI and epsilon caprolactam, the reaction cannot be controlled in such a way that 1 mole of IPDI always reacts only with 1 mole of epsilon caprolactam. To the extent that diblocked IPDI forms, free IPDI remains, i.e., the reaction product of 1 mole of IPDI and 1 mole of epsilon caprolactam (and this basically applies to all diisocyanates and their conversion with monofunctional blocking agents) is always a mixture of IPDI monoblocked with epsilon caprolactam, IPDI diblocked with epsilon caprolactam, and free IPDI.
Thus, there is a need for a process to produce monoblocked diisocyanates with only a slight formation of free diisocyanate and diblocked diisocyanate.
There is a further need for a process for the production of monoblocked cycloaliphatic diisocyanates with only slight formation of free diisocyanate and diblocked diisocyanate.